Protective circuit



March 2, 1943. F, KURTZ 2,312,915

PROTECTIVE CIRCUIT Filed Sept. 15, 1939 2 Sheets-Sheet 1 Z2 23 M-l March2, 1943. E. F. KURTZ 2,312,915

PROTECTIVE CIRCUIT Filed Sept. 15, 1939 2 Sheets-Sheet 2 FIG.3.

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4 'Mdld Patented Mar. 2, 1943 PROTECTIVE CIRCUIT Edward F. Kurtz,Attleboro, Mass, assignor to Metals & Controls Corporation, Attleboro,Mass, a corporation of Massachusetts i Application September 15, 1939,Serial No. 295,039

6 Claims. (Cl. 175-294) This invention relates to protective circuits,and with regard to certain more specific features, to electricaloverload protective circuits for energy translating devices such asmotors, generators, transformers, electromagnets, and the like.

Among the several objects of the invention may be noted the provision ofprotective circuits provision of circuits of the class described inwhich only the indicated thermally responsive switches are required(supplem ntary relays, magnetic switches and the like beingunnecessary); the provision of circuits of the class described whereineach 01. said thermally responsive switches performs a protectivefunction govemed by the current in an individual, different supply wire,or phase, but in which the openings of any one such switch disconnectsthe entire energy translation device from its power supply; theprovision of circuits of the class described requiring closing of all ofthe separate thermally responsive switches in order to energize theenergy translating device; and the provision of circuits 01' the classdescribed which are relatively simple and employ simple and economicaldevices therein. Other objects will be in part obvious and in partpointed out hereinafter.

The invention accordingly comprises the ele ments and combinations ofelements, features of construction, and arrangements of parts which willbe exemplified in the structures hereinafter described, and the scope ofthe application of which will be indicated in the following claims.

In the accompanyingv drawings, in which are illustrated several ofvarious possible emhodh ments of the invention,

Fig. 1 is a diagram of a circuit embodying the For the sake ofsimplicity herein, the invention is described as it applies to a motor,but it will be obvious to anyone skilled in the art that the inventioncan be similarly and equally well applied to generators, transformers,electromagnets and other forms of energy-translating devices.

There is a growing tendency to use thermaloverload protectors inconjunction with electrical motors, which control the motor directly(without intervening relays or contactors) and are of such nature thatthey can be mounted directly in the motor by the motor manufacturer. Inthis way, the motor manufacturer can be assured that theoverload-protection switches have the proper characteristics for theparticular motor; and because the protector is responsive to the actualtemperature of the motor, more accurate protective results can beobtained. One of the problems in this art has been the protection oi?polyphase motors in a simple and eillcient manner.

It has sometimes been the practice hereto fore, for example, on smallthree-phase motors, to use two protectors each comprising a singlepoleswitch, a thermally responsive element adapted to operate said switch,and a heater in series with said switch and thermally adjacent to saidelement; one such switch being connected in each of two terminal leads.Several forms of such protectors are described in detail in John D.Bolesky applications Serial Nos. 158,187 and 189,075, filed August 9,1937, and February 7, 1938, respectively. But this method givesunsatisfactory performance for three-phase motors although it does serveto protect the motor. For example, unless the heating of the motor atthe points of mounting of the two protectors is the same, unless thecurrents in the two terminal leads in which the protectors are mountedare exactly'the same for the two protectors, and unless thecharacteristics of the two protectors are exactly the same, 'one willprecede the other in functioning. Since absolutely uniform conditionsare practically unknown in actual practice, this nomsimultaneousfunctioning usually happens, and the motor consequently operates singlephase until the other protector functions. ihe first protector to openwill probably reclose first if it is of the self-resetting type and themotor will remain stalled as it cannot start on single phase. Thus it islikely that the heavy stalled current will again trip this protectorbefore the second resets and the two will continue, on and ofi, untilthey accidentally fall into step long enough to restart the motor. Thismeans indeterminable delay. To overcome this the manual-reset type ofprotector as shown in said patent application Serial No. 189,075-may beused, but unless mechanical arrangements are provided so thata singlepush button operates both protectors, two buttons will have to be pushedeach time, and, furthermore, manual reset protectors cost considerablymore than automatic reset types, and the majority of applicationsarebetter served by the automatic type. To meet these conditions it hasheretofore sometimes been the practice to interlock mechanically twoprotectors so that they operate mechanically in unison, or at least sothat one may not reset while the other remains open. But such mechanicalinterlocking, while giving the desired performance, is more costly thanthe type of protectors and method connecting herein described. Also,there are various other disadvantages in the mechanical interlocking, assuch protectors must be specially made for polyphase use, requiregreater skill in design and manufacture, and are naturally morecomplicated.

In the present invention, two thermal switches are used, each of whichhas a single-throw, double-pole switch actuated by a single thermalelement, and the two switches are electrically connected in a circuitthat accomplishes the above objects.

While it is not a part of the present invention, the construction of aswitch which will satisfactorily accomplish the proper circuitoperations, according to the invention, will first be described.

Referring to Figures 2, 3, and 4, numeral i indicates a base ofelectrical insulating material, upon which are mounted the variouselements of the switch. A supporting plate 2 made of brass or steel orother electrical-conducting material is held on the base i by means oi arivet 3 passing therethrough. Suitably attached to supporting plate 2 atone end thereof by welding, riveting or any other suitable fastening, isa thermostatic disc 4 such as, described and claimed in J. A. SpencerPatent 1,448,240, dated March 13, 1923. Disc 4 is shown electricallyspot-welded to the supporting plate 2 by means of a tab 5 formed on thedisc as an integral part thereof. A supporting and adjusting screw 5 isfastened to the center of the disc in a rotatable manner by means ofproper shoulders and abut-= ments as indicated at numeral i, and, byen-=- gagement with a threaded hole 8 in the base I,

is used to adjust the operating temperatures of the element It. A locknut 9 is used to hold screw 6 from turning after the adjustments aremade. Welded or otherwise suitably fastened to the diametricallyopposite point on the disc from the tab 5 is a contact button In. Buttonl0 makes good electrical connection with the disc 4. Cooperating withthe button it! is a contact plate II which is held firmly on the base iby means of a rivet l2.

Extending through base i in a slldable manner in a hole therethrough' isa push rod it which is actuated by the motion of the thermal element 4,and which is made of an electrical in= sulating material such asBakelite. Push rod l3 in turn moves a switch arm II on the other side ofthe base..' Switch arm i4 is made of spring steel or brass, or any othersuitable electrical conducting material having the necessary resilientcharacter. Switch arm it is mounted motor.

firmly at one end on the base by means of a rivet l5 passingtherethrough. Fastened on the opposite or movable end of the switch armit by welding or riveting so as to be in electrical con= nectiontherewith is an electrical contact button l6. Cooperating with movablecontact button I6 is stationary contact plate H'- which is held securelyto the base i by means of a rivet l8. Spring arm I is tensioned slightlyto hold contacts l5 and i! normally engaged. In a recess l9 provided inthe base i is mounted a heater wire 20 adapted to heat the thermalelement 4. One end of heaterwire 20 is connected to contact plate i Iand the other end to a terminal plate 2!. The purpose of the heater 28is to offset the lag in temperature between the winding of the motor andthe thermal element a when the winding temperature rises rapidly from excessive current as when the motor is stalled or very heavily overloaded.

Other details of the construction of the switch need not be recitedherein. The operation of the switch is as follows:

If the disc 4 is heated either by increase in ambient temperature, or bythe passage of electrical current through itand the electrical heater20, or by a combination of these means, to a tern perature suiiicient toactuate it, it will snap its movable end and contact ID to the rightfrom the Fig. 3 position, for reasons indicated in the abovementionedSpencer Patent 1,448,240. In doing so, the disc 4 breaks the electricalconnection between movable contact it and stationary contact ii. At thesame time, it also pushes to the right the insulating push rod is, whichin turn pushes the switch member I 4 to the right, thus breaking theelectrical connection between movable contact i5 and stationary contactll. Thus, it is seen that the single thermally actuated element 4aotuates what is in eiiect a single-throw, double-pole switch.

Any'other switch of suitable nature can be used in the circuits of thepresent invention.

Referring now to Fig. 1, there is shown in schematic form a circuitembodying the present invention. Numerals 22, 23, and 25 each indicatethe power leads of a polyphase source of electrical current supply.Indicated at numeral 25 is a line switch controlling the motor andrepresenting either a manual or an automatic switch of the type commonlyused to start and stop the The two thermostatically actuated pro= tectorswitches of the type heretofore described are indicated at letters A andB and comprise the discs shown schematically as at la and ab, the switcharms shown schematically as at its and Nb with the contacts iiia, iia,ita and its in the motor winding 29; and terminal plate Zib is connectedby a wire 32 to terminal 33 of the motor winding 29'.

Considering the circuit connections for each of the motor windingterminals:

For terminal 28, current passes from the power supply through wire 22,thermostatic disc heater 20a, and wire 2?. Forterminai 33, cur

pisses through power line 24, thermostatic eater and wire 32. Forterminal 3!, asses through power line 23, switch arm Ell, switch arm Mo,and wire 30.

or the protective circuit is as otectors are made to match the motoriteintics properly, and are properly mount-= or in "the motor, thecurrents traversing .ters and discs plus the ambient temture within themotor where the protectors will cause the discs to open when maximumsafe temperature for the motor line is reached under any operatingcondi- It is not likely that the discs will function simultaneously, buteither one on functioning interrupts two of the three wires 22, 23, and24 suppl ing power, and. hence either deenergiaes the motor. Similarly,if both switches should cut out, ,he motor will not be re energizeduntil the last does so, either automatically or manually depending uponthe particular type of switch emplayed.

Fig. 7. is shown a delta connected threewinding 29 for the motor ordevice prod. Eiowever, the circuit shown serves equally if athree-phase, Y-connectecl winding 34 c) or a two-phase, three-wirewinding 35 is substituted. ..e principle of this invention can b equallywell applied in other types of polyphase systems as the two-phase,four-wire system shown in 5, which one protector switch (A and B) iscomiected in series with each of two windings "-d with the extra pole ofeach switch (A in series with the other winding 31 or 36,

c.iown in Fig. 5, numerals 38, 39, 40, and 4| icating the four powerlines. n referring to Fig. l, the circuit is likewise operative if theswitch pole represented by conand i "la were connected in series in wireor if the pole represented by contacts 16b were connected in series inwire 2?. In act, it is immaterial in what relationship the two tones areconnected except that obviously the eaters Ella Elli; should not be inthe same e a heater Zea (or 2012) is desirable for types or" illlDbDlS,it is not needed forsome oi energy-translating devices, particularly osewhich only gradual heating can occur.

.law or the above, it will be seen that the veral objects of theinvention are achieved and r advantageous results attained.

many changes could be made in the above tructions without departing fromthe scope of e invention, it is intended that all matter con- "ive andnot in a limiting sense.

I I in combination with an electrical polyphase translating devicehaving at least three t minals, a protective circuit therefor doublepole switches each having ly responsive means for actuating it, and

at least three power supply lines for the device, one of said linesbeing connected to one of said power terminals with one pole of oneswitch in series therewith, a second line being connected. to a secondpower terminal with one pole of the other switch in series therewith,and the second pole of each of said switches being connected in one ofthe lines other than the line in which its respective first pole isconnected.

2. A circuit as set forth in claim 1, including electrical heating meansin each of the said switches adapted to afiect their respectivethermally responsive means in accordance with the electrical currentpassing through said heating means, the respective heating means foreach switch being connected in series in one of the power linesconnected through the switch it affects but not in the same power linewith the other heating means.

3. A circuit as set forth in claim 1, including electrical heating meansin each of the said switches adapted to afiect their respectivethermally responsive means in accordance with the electrical currentpassing through said heating means, the respective heating means foreach switch being connected in series in one of the power linescontrolled by that switch.

4. In combination with. an electrical polyphase energy translatingdevice having two windings with two power terminals each, a protectivecircuit therefor including two double pole single throw switches eachhaving thermally responsive means for actuating it, and four powersupply lines for the device, one of said power lines being connected toone terminal of the first winding with one pole of the first switch inseries therewith, a second nected to the other terminal of said firstwinding with one pole of the second switch in series therewith, a thirdpower line being connected to one terminal of the second winding withthe other pole of the first switch in series therewith, and the fourthpower line being connected to the other terminal of the second windingwith the other pole of the second switch in series therewith.

5. A circuit as set forth in claim 4, including electrical heating meansin each of the said switches adapted to affect their respectivethermally responsive means in accordance with the electrical currentpassing through said heating means, the respective heating means foreach switch being connected in series inone of the power linescontrolled by that switch.

6. In combination with an electrical polyphase energy-translating devicehaving at least three power tedminals, a protective circuit thereforincluding two double-pole switches each having thermally responsivemeans for actuating it, and at least three power supply lines for thedevice, one of said lines being connected to one of said power terminalswith one pole of at least one switch in series therewith, a, second linebeing connected to a second power terminal with one pole of the otherswitch in series therewith, and the second pole of each of said switchesbeing connected in one of the lines other than the line in which itsrespective first pole is connected.

EDWARD F. KURTZ.

power line being con"

